FIELD OF THE INVENTION
The present invention concerns improvements to wheel suspensions and in particular to steerable wheels of land vehicles and especially to those ensured by two lateral arms situated on the same side of the wheel.
Such devices are known in the prior art and in particular in French patent published under n.degree. 2,418,141 (Andre de Cortanze).
According to this patent, the front suspension has the general form of a quadrilateral of which the apices are the ends of two arms, pivoted at one of their ends to an element integral with the vehicle body and at the other end to a carrier triangle for the stub axle of the front wheel.
The axis corresponding to the side of the said triangle joining the ends of the arms allows, turning the-front wheel. It is realized by two pivoting and hinging devices with the ends of the arms of the swivel type or equivalent. The two arms are rotatively mounted on the vehicle body through two substantially horizontal axes (in normal position of the vehicle) and they are situated substantially in a single vertical plane thus containing the center of the swivel joints and the steering axis.
The shock absorbing and suspension system per se is essentially constituted by a spring/shock absorber assembly generally disposed between an arm and an element integral with the vehicle body. When the suspension functions, the arms pivot in the said vertical plane, provoking the displacement in this same plane of the swivel joints and thus of the steering axis.
With respect to the steering system itself, it is constituted by a side lever integral with the triangle moved by a kinematic train transmitting to the said lever the movements of the handle bar or other steering control device.
According to a novel solution which is the object of a patent application filed the same day as the present application by the applicant and having for its title "Improvements to steerable wheels of land vehicles" the two arms carry at their end a tetrahedric piece one apex of which is integral with the stub axle of the wheel, and three apices correspond to the centers of universal joints such as swivel joints, cardans or equivalent of two of them are pivoted to the suspension arms an define the steering axis, the third being pivoted to the kinematic train for steering control and thus for turning the wheels.
In the following description, reference will be made to this latter solution in order to illustrate the present invention, it being well understood that it applies to all suspensions with two arms.
It will furthermore be recalled that in most former systems, it has been sought to improve the stability and especially to avoid the phenomenon which is called "dive" and sometimes "leap".
Upon cycles and in particular motorcycles, but also on a number of other vehicles and in particular automobiles, a braking on the front wheel provokes a shifting torque towards the front so that the front suspensions are compressed while the rear suspensions are generally unloaded. This is increasingly clear since most frequently it is rendered apparent in a lowering of the front of the vehicle and the subsequent increase of the torque effects. Thus, on most motorcycles with conventional suspension having a fork and shock absorber and springs working along the length of the fork, the compression of the suspension is rendered evident by a shortening of the fork while the rear is in raised position.
On this type of vehicle with conventional suspension, the relative movements of the front and rear wheels are relatively easy to define, the axles describing straight lines or sometimes arcs of circles so that the dive or the anti-dive result from simple geometric arrangements and from definitions, there also relatively simple, of the static and dynamic rules applicable to the suspensions.
In the case of systems with two lateral arms or more than two arms, the geometric arrangements require more complex rules, so that the general reactions of a vehicle, for example in the case of braking, are relatively more difficult to foresee.
In fact, if reference is made to FIG. 1 representing very schematically a conventional bicycle front suspension without specific suspension, the axle is essentially subjected to two forces during braking, a reaction force P to the gravity (the weight of the loaded vehicle is statically distributed between the axles) and a reaction force F to braking (corresponding to deceleration). Upon halt or at constant speed in a straight line, F is nil and only P intervenes. In this description, the cases of cornering introducing, of course, complementary forces and especially the centrifugal force and the corresponding reaction will not be considered.
If the resulting R.sub.1 (for the braking F.sub.1) passes under the center of gravity G of the loaded vehicle, the corresponding torque tends to shift G towards the front and the motorcycle "dives" or "leaps". If this component R.sub.2 (for the braking F.sub.2) passes above G (which is the case of slight or nil braking) it goes in the opposite direction. FIG. 1 represents a component R.sub.2 coinciding with the reaction in the fork.
When the situation according to FIG. 2 occurs with a front fork suspension, the case of most motorcyclettes, the resulting R will be divided into a force S provoking the deformation of the suspension and a component C which passing under the center of gravity G corresponds to a shift torque towards the front. In these cases, it is not possible to oppose to the dive provoked by the braking.
When the vehicle has a suspension that is geometrically more complex and in particular suspension with two arms or more, the determination of the dive or antidive conditions itself becomes very complex, so that it is difficult to define the geometries that are opposed or not to the dive. The applicant has been able to define simple conditions which, when they are satisfied by geometric definitions of the suspensions, ensure the antidive.
In the following description, are considered the positions and the movements taken relatively with respect to the body vehicle supposed to be fixed and suspended, in a position corresponding to that which it would have in lying on the ground with a mean load.
Considering in the longitudinal axial plane of the vehicle, the curve described by the axis of the wheel when the suspension functions, the applicant has observed that when the horizontal distance from the axis to an given point of the vehicle body remains constant or increases when the at least apparent load on the said axle increases, the suspension is inhibiting the dive of the vehicle whatever the type of suspension; this has a very interesting application in the case of the system with two bars or more for which the described curves are complex.
It will be recalled in particular that in a mechanical system where a plane P.sub.1, (for example, that of the axle and of its bracket) is displaced with respect to a plane P.sub.0 (that of the vehicle body) through a kinematic device (the two arms in the present example), the instantaneous center of rotation of P.sub.1 with respect to P.sub.0 describes in the plane P.sub.0 a curve b.sub.0 called base, and in the plane P.sub.1 a curve r.sub.1 called rolling which, during the movement of P.sub.1 on P.sub.0, rolls without sliding on the base b.sub.0, each fixed point of P.sub.1 (and in particular here at the axle 0) describing a curve of P.sub.0 called roller r.sub.0. What is important therefore is the roller r.sub.0 described by the axle 0 in the plane P.sub.0 of the vehicle body.
If the rule set out herein-above is applied, the at least apparent load increase is rendered evident by a displacement of 0 towards the top along the roller r.sub.0 in the plane P.sub.0 .
If 0 is displaced towards the rear (in the case of the front wheel), i.e. towards the right of the figures, the suspension authorizes the dive. If 0 goes towards the front or remains at the same abcissae (with respect to the horizontal) there is no dive and the greater the displacement towards the front (on the left of the figures) the more the suspension presents an anti-dive behavior.
The tangents to the roller described by the axis of the wheel must therefore be at least vertical and preferably inclined downwards to the rear, towards the top in the front so that when the axis 0 climbs with respect to the vehicle body, it moves away towards the front.
In this description of the examples, reference will be made to steerable wheels suspension that raises the problem of steering. But, it is obvious that the appropriate suspension for a wheel that turns is a fortiori suitable for a non steerable wheel.